Abstract: Boiling is a very complex heat transfer process. According to experiments, bubbles produced at one nucleation site have different sizes and the mean bubble departure diameter is even different at different nucleation sites under same working conditions. Thus, the distribution of bubble size cannot be ignored. Nowadays, a lot of semi-empirical correlations were proposed in predicting bubble departure diameter, but the study of bubble size distribution is rare. Therefore, this paper focused on the bubble size distribution. Firstly, the accuracy of Gamma distribution and Gaussian distribution was compared based on the experimental data. Then, two determined parameters, such as average and standard deviation were analyzed respectively. Finally, a set of bubble departure diameter distribution correlations for flow boiling was obtained which could reflect the distribution of bubble departure size in different mass flux, wall superheat.